Biology 30S THE CIRCULATORY SYSTEM Name: This module adapted from bblearn.merlin.mb.ca 1
Introduction to Circulation The first organ to form, and the last organ to die. The heart is the pump of life. The main functions of the circulatory system: Types of circulatory system in animals: Cardiovascular disease accounts for the death of more Canadians than any other disease. In 1999, cardiovascular disease accounted for 78,942 Canadian deaths. 35% of all male deaths in Canada in 1999 were due to heart diseases, diseases of the blood vessels and stroke. For women, the toll was even higher 37% of all female deaths in 1999 were due to cardiovascular disease. Why do you think Cardiovascular related deaths are so high in Canada? The circulatory or cardiovascular system plays a vital role of maintaining homeostasis in the human body. This homeostasis depends on the continuous and controlled movement of blood through the thousands of kilometres of blood vessels that ultimately reach every cell in the body. It is in the microscopic blood vessels that blood performs its ultimate transport function. Nutrients and other essential materials pass from blood into fluids surrounding the cells and waste products are removed. In this module, you will study the structure and function of the human circulatory system including the heart and the lymphatic system. You will also study and research some of the disorders and diseases that affect this system. 2
Introduction to Lesson 1 All living things require nutrients to grow and reproduce. Single celled organisms and very simple multicellular organisms are able to diffuse these nutrients from the environment into their cells. As organisms become more sophisticated (more than two cell layers thick), a transport mechanism is required to transport nutrients to the cells and transport wastes away from cells. In humans, the circulatory system performs this transport function. Lesson 1 Outcomes By the end of this lesson, you should be able to: -List six ways in which the circulatory system maintains homeostasis in the human body. -Explain how the structure of the five different types of blood vessels (arteries, arterioles, veins, venules, capillaries) is related to their function. -Describe how the structure of the heart is related to its function, i.e. double pump. -Identify and trace blood flow through the following structures of the heart from a specimen, model, or diagram: left and right atria left and right ventricle left and right pulmonary arteries left and right pulmonary veins superior/inferior venae cavae septum aorta left and right semilunar valves left and right atrioventricular valves 3
-Describe the difference between the systemic and pulmonary circulatory system. -Identify the following systemic blood vessels from a specimen, model, or diagram: carotid arteries jugular veins subclavian artery and vein superior/inferior venae cavae coronary artery and vein renal artery and vein iliac artery and vein hepatic portal vein Lesson 1 Overview Following is a list of topics covered in this lesson. The Circulatory System and Homeostasis Blood Vessels The Heart Major Systemic Blood Vessels Fetal Circulation The Circulatory System and Homeostasis The human circulatory system (also known as the cardiovascular system) consists of the heart, which is a muscular pumping device, and a closed system of vessels that are known as arteries, arterioles, veins, venules and capillaries. Blood contained in the circulatory system is pumped by the heart around a closed circle or circuit of vessels as it passes again and again through the various "circulations" of the body. The circulatory system is vital to the maintenance of homeostasis in the body. Maintaining homeostasis depends on the continuous and controlled movement of blood through the thousands of kilometres of capillaries that permeate every tissue and reach every cell in the body. It is in the microscopic capillaries that blood performs its transport function. 4
The circulatory system performs a number of important homeostatic functions in the human body. These include: Blood Vessels - Arteries Blood Vessels Blood vessels are the channels through which blood is distributed to body tissues. The vessels make up two closed systems of tubes that begin and end at the heart. These two systems are: Arteries Characteristics: The aorta, the largest artery in the human body, is about 25 mm in diameter. Arterioles on the other hand are only about 0.2 mm in diameter. 5
Artery walls are thick, strong and muscular and made of three tissue layers. In large arteries the middle layer is made mostly of elastic fibers. These fibers allow the large arteries to accommodate surges of blood pumped by the heart. The thick elastic walls stretch with the changing blood flow. This property of elasticity means that they can expand to accept a volume of blood, and then contract and squeeze back to their original size after the pressure is released. A good way to think of them is like a balloon. When you blow into the balloon, it inflates to hold the air. When you release the opening, the balloon squeezes the air back out. As the arteries get smaller, the blood pressure gets less and the need for elastic fibers in the middle section diminishes. This layer becomes mostly muscle fibers that contract, changing the size of the arterial channel, regulating the pressure and amount of blood that enters the capillaries. The artery's outer layer is made of fibrous, connective tissue, nerves, and tiny blood vessels that nourish the artery's walls. The diagram below illustrates the structure of arteries. Figure 5.1.1 Structure of Arteries (http://training.seer.cancer.gov/module_anatomy/unit7_3_cardvasc_blood1_classificatio n.html 6
Capillaries and Veins Capillaries Characteristics: The average diameter of a capillary is 7/1000 mm (7 µm), just wide enough to let red blood cells pass through single file. Capillaries are really more like a web than a branched tube. It is in the capillaries that the exchange between the blood and the cells of the body takes place. Here the blood gives up its oxygen and takes on carbon dioxide. In the special capillaries of the kidneys, the blood gives up many waste products in the formation of urine. Capillary beds are also the sites where white blood cells are able to leave the blood and defend the body against harmful invaders. The diagram below illustrates a capillary bed. Figure 5.1.2 Capillary Bed (http://training.seer.cancer.gov/module_anatomy/unit7_3_cardvasc_blood1_classificatio n.html) There are approximately 100 000 km of capillaries in an adult. Because these minute capillaries are so numerous, (about 10 billion) they present a huge surface area to the tissue. More than 800 square meters of surface area allows for a great deal of exchange between the blood and the tissues. Tissues are so permeated with capillaries that rarely are any cells more than one cell layer away from a capillary. 7
This is necessary to allow for the diffusion of materials between the body cells and the walls of the capillaries. The walls of capillaries are only one cell thick. Substances in the blood and substances in body tissue are exchanged only across the capillary endothelium. Capillary networks serve nearly all of the living tissue of the body, their concentration in the tissue depending on the local need for exchange of materials. Muscles, which are called upon frequently to move the body, and the kidneys, which must remove waste products constantly, require great quantities of food and oxygen and are well supplied with capillaries. On the other hand, the cornea of the eye, a very inactive tissue, has none. Veins Characteristics: Because the walls of the veins are thinner and less rigid than arteries, veins can hold more blood. Almost 70 percent of the total blood volume is in the veins at any given time. Although the blood is forced into the arteries under pressure, by the time it reaches the veins, this pressure is very low. Blood pressure in the veins is less than 1/10 of the pressure in the aorta. Therefore, another mechanism must be present for getting blood back to the heart. How do veins function in moving the blood despite the low pressure? 8
The diagram below illustrates the structure of veins. Veins, like other blood vessels are subject to problems. What is varicose veins? The Heart One of the first important events of your life took place about three and one-half weeks into your embryonic development. Your heart began to beat. You may be excused for not remembering as you were only about 2.5 mm in size at the time. Your heart now is about the size of a large fist and has a mass of approximately 300 grams. It is a tough muscular organ which beats about 70 times and pumps 5 liters of blood every minute. Pumping over 7000 liters of blood each day, it has pumped about 35 million liters in your life time as a grade eleven student. While most of the hollow organs of the body do have muscular layers, the heart is almost entirely muscle. Unlike most of the other hollow organs, whose muscle layers are composed of smooth muscle, the heart is composed of cardiac muscle called the myocardium. The heart is surrounded by a fluid-filled membrane called the pericardium.the pericardial fluid bathes the heart, preventing friction between its outer wall and the membrane. 9
The human heart is really two pumps working side by side. A thick wall of muscle, called the septum, separates the heart's right and left sides. Each side is divided into two chambers: the atrium and the ventricle. The upper chambers, the left atrium and right atrium, collect blood returning to the heart through veins. The thin muscles of their walls push blood a short distance into the lower chambers, the left ventricle and right ventricle. The thick, muscular walls of the ventricles contract forcefully, pushing blood out of the heart to the lungs and body through arteries. The heart is responsible for pumping the blood to every cell in the body. It is also responsible for pumping blood to the lungs, where the blood gives up carbon dioxide and takes on oxygen. The heart is able to pump blood to both regions efficiently because there are really two separate circulatory circuits with the heart as the common link. Some even refer to the heart as two separate hearts, a right heart (pulmonary system) and left heart (systemic system). In the pulmonary system, blood leaves the heart through the pulmonary trunk which branches into the left and right pulmonary arteries, goes to the lungs, and returns to the heart through the left and right pulmonary veins. In the systemic system, blood leaves the heart through the aorta, goes to all the organs of the body through the systemic arteries, and then returns to the heart through the systemic veins. Arteries carry blood away from the heart and veins carry blood toward the heart. Most of the time, arteries carry oxygenated blood and veins carry deoxygenated blood. However, there is an exception. The pulmonary arteries leaving the right ventricle for the lungs carry deoxygenated blood and the pulmonary veins carry oxygenated blood. The diagram below illustrates this relationship. Figure 5.1.4 Pulmonary Circulation(http://training.seer.cancer.gov/module_anatomy/unit7_3_cardvasc_blood3_ pathways.html) 10
Blood from any body tissue other than the lungs returns to the heart through either of two veins: superior vena cava and inferior vena cava. Refer to the diagram below. Figure 5.1.5 Blood Flow though the Heart (http://www.tmc.edu/thi/anatomy.html) 11
A simplified diagram of the blood flow: Two valves regulate the flow of blood between the atria and ventricles. These valves, commonly called the atrioventricular (AV) valves, consist of three flaps of tissue that together form a more or less funnel-shaped arrangement, the narrow end extending into the ventricle. The pressure of the blood in the atrium forces the valve open, but when pressure develops in the ventricle, the pressure pushes the flaps against each other, effectively closing the opening. These two valves that regulate blood flow between the atria and ventricles are - Tricuspid valve: Mitral valve: Two valves regulate the flow of blood between the ventricles and the major vessels leaving those ventricles. These valves are commonly known as semilunar valves. The two semilunar valves are Pulmonary valve: 12
Aortic valve: The branches of the aorta carry oxygenated blood to all parts of the body except the lungs. In the brain, a muscle, a gland, or some other organ, the oxygenated blood becomes deoxygenated blood as it releases its oxygen and accepts carbon dioxide from the tissues. Systemic and Coronary Circulation Major Systemic Arteries and Veins All systemic arteries are branches, either directly or indirectly, from the aorta. The aorta ascends from the left ventricle, curves to the left, and descends through the thorax and abdomen. This geography divides the aorta into three portions: After blood delivers oxygen to the tissues and picks up carbon dioxide, it returns to the heart through a system of veins. The capillaries, where gas exchange occurs, merge into venules and these converge to form larger and larger veins until the blood reaches either the superior vena cava or inferior vena cava, which drain into the right atrium. The diagram below illustrates the major blood vessels in the human body. 13
Figure 5.1.6 Major Blood Vessels (http://users.rcn.com/jkimball.ma.ultranet/biologypages/c/circulation.html) Coronary Circulation Function: There are two main coronary arteries, the right and left coronary arteries with two major branches each. They arise from the aorta right after it leaves the heart. The coronary arteries eventually branch into capillary beds that are embedded throughout the heart walls and supply the heart muscle with oxygenated blood. The coronary veins return blood from the heart muscle, but instead of emptying into another larger vein, they empty directly into the right atrium. 14
Figure 5.1.7 Coronary Arteries (http://www.tmc.edu/thi/coroanat.html) Disease in coronary arteries prevents the heart from receiving enough oxygen. These diseases will be discussed in lesson 5. Fetal Circulation In the human fetus, the lungs are not functional; the placenta substitutes for the lungs as the organ of gas exchange. How does the fetus gas exchange work? Oxygenated blood is delivered to the fetus from the placenta by the umbilical vein. This highly oxygenated blood flows into the inferior vena cava, which enters the right atrium. However, deoxygenated blood being returned from the internal organs contaminates the pure placental blood flowing in the inferior vena cava. Fortunately, the volume of placental blood is large, so that the mixture entering the right atrium is relatively well oxygenated. Ordinarily, blood would flow directly from the right atrium into the right ventricle, and, in turn, would leave the heart through the pulmonary trunk to the lungs. This would be a useless course in the fetus since the lungs are inactive. 15
The main volume of the relatively pure blood in the right atrium crosses through a special opening, known as the foramen ovale, into the left atrium. From the left atrium the blood reaches the left ventricle, which pumps the blood into the aorta to be delivered through the systemic system. Thus, the foramen ovale is an important device to ensure that a considerable portion of the oxygenated blood passes directly from the right atrium into the left atrium. Blood that passes from the right atrium to the right ventricle will be directed through a second shunt, called the ductus arteriosus that leads to the aorta. Some of the blood will reach the lungs through the pulmonary trunk, but a greater part arising from the right ventricle will continue through the ductus arteriosus to the aorta. Fetal circulation ceases at birth. When the lungs of the newborn expand with air, pulmonary circulation begins so that there will be an adequate supply of oxygen to the body. Constriction of the ductus arteriosus occurs shortly after birth with the result that the blood leaving the right ventricle no longer bypasses the lungs. Also, the foramen ovale is gradually sealed. What is a Blue Baby? 16
Lesson 1 Exercise 1. List 5 ways that the human circulatory system contributes to homeostasis. 2. Compare and contrast the structure and function of arteries, capillaries and veins. 3. Describe main differences between arteries and veins in terms of structures, functions and locations. 4. Why is it necessary to have 100 000 km of capillaries in your body? 5. Differentiate between the pulmonary and systemic circulatory systems. 6. Name and give the function of the four valves found in the heart. 7. Using the following diagram of the heart on the next page, label the major structures and trace the flow of blood using arrows. Use the colors blue and red to distinguish between deoxygenated and oxygenated blood. 17
8. How does the heart receive its supply of blood? 9. What two modifications to the circulatory system are found in developing embryos? 18
Lesson 1 Summary In this lesson, I learned: 19